In history class, the girl to boy ratio is $3$ to $9$. If there are a total of $36$ students, how many girls are there?
A ratio of $3$ girls to $9$ boys means that a set of $12$ students will have $3$ girls and $9$ boys. A class of $36$ students has $3$ sets of $12$ students. Because we know that there are $3$ girls in each set of $12$ students, the class must have $3$ groups of $3$ girls each. There is a total of $9$ girls in history class.